hmm, too bad I can't go back to that one now but I have noticed something. It seems to me that some sentences that are using gerund, like He is eating or They are drinking, does not accept sentences with 'aan het' as a translation rather they just translate it to a more simple sentence. Like Hij eet of Zij drinken but in english it is mostly translated to as he eats or they drink.

It depends what kind of sentence it is, if it is something that is currently happening, like we are reading (a book now) it can be translated either as we lezen (nu een boek) or we zijn (nu een boek) aan het lezen both are fine, the second is putting more emphasis that it is currently happening.

However sentences like I enjoy playing (a fact or habit) cannot be translated using aan het, only as Ik hou van spelen.